An iris image definition estimation system using the astigmatism of the corneal reflection of a non-coaxial light source to assess both the resolution of the iris patterns and the direction of focus adjustment. The corneal reflection results in two virtual images on the meridional and the sagittal planes. These virtual images are formed behind the cornea and close to the iris. Yet, both are projected onto the same location and result in a composite glint area. In addition, the shape of the glint area of a non-coaxial light source varies with different camera focus settings. Furthermore, due to the high intensity of the glint area, the shape can be easily observed, and the size and the shape of the glint area can be used to determine the resolution of the iris image and the direction of focus adjustment, respectively.